1. Field of the Invention
The present invention relates to an air flow rate measuring device mounted on an intake pipe of an internal combustion engine for measuring the flow rate of air flowing through a main passage inside the intake pipe.
2. Description of the Related Art
In the past, an air flow rate measuring device is used to measure the flow rate of air flowing through a main passage of an intake pipe of an internal combustion engine, and in recent years, demands have been made for improvements in the accuracy of measurements in a low flow rate range as well as at the time of transition in accordance with an increased range of intake flow rate measurements accompanying the increased output power of the internal combustion engine and the tightening of emission control.
As one example of such an air flow rate measuring device, the one described in a first patent document (Japanese patent application laid-open No. 2003-35578) for example includes a flow rate detection element and electronic circuit parts which are mounted on a rectangular-shaped support substrate made of a ceramic material or the like at positions different from one another in the longitudinal direction thereof. In addition, it also includes a housing in which electronic circuits implemented on the support substrate are received, a flow conduit in which coolant or cooling fluid is caused to pass in the primary direction of flow in a main passage so as to cool the support substrate, and an auxiliary flow conduit with auxiliary passages through which the cooling fluid is caused to flow in a direction opposite the primary direction and in which the above-mentioned flow rate detection element is arranged.
In such a known air flow rate measuring device, the support substrate is held in the central portion of the auxiliary passage. Accordingly, there arises a problem that positional displacements and/or angular deviations of the support substrate with respect to the flow of air can be caused easily, resulting in reduction in the accuracy of measurements.
Moreover, component parts for holding the flow rate detection element in position and component parts for forming a passage to introduce air into the flow rate detection element are required individually and separately, thus posing another problem. That is, the number of parts required is increased and hence the assembly man-hour required accordingly becomes a lot, and the accuracy of measurements is reduced owing to dimensional variations and assembly variations of the parts.
Furthermore, since the support substrate with the flow rate detection element mounted thereon is arranged in the central portion of the auxiliary passage, there is an additional problem, too, in that the pressure loss in the auxiliary passage is increased by the support substrate.